1. Technical Field
The present invention relates to a spark plug for an internal-combustion engine.
2. Related Art
A spark plug, which provides an ignition means for an automobile engine or the like, includes a spark gap between a central electrode and a ground electrode confronted with each other. Application of a pulsed voltage between the central electrode and the ground electrode generates a discharge.
There is known such a spark plug that a noble metal tip is joined to each of the central electrode and the ground electrode. As an outer diameter of either or both of these noble metal tips decreases, the ignitability increases, whilst the attrition resistance decreases. In addition, as a protrusion of the ground electrode side noble metal tip increases, the ignitability increases, while leading ends of the noble metal tips become hot, thereby reducing the attrition resistance.
To improve the attrition resistance of the noble metal tips while ensuring the ignitability, techniques as disclosed in Japanese Patent Application Laid-Open Publication No. 2005-123181 use noble metal tip materials that have high temperature oxidation resistance to satisfy a predetermined condition.
However, the disclosed techniques have limited choices in noble metal tip materials, which may lead to an expensive spark plug.
In recent years, there is a trend for a flow rate of an air stream, such as a tumble flow, in a combustion chamber to get higher as an internal-combustion engine is scaled to high powers and lean combustions. A discharge spark generated between the central electrode and the ground electrode is prone to be blown out at a higher flow rate of the air stream in the combustion chamber, which leads to a higher frequency of re-discharge. This leads to an increased amount of wear of the central electrode and the ground electrode (hereinafter merely referred to as electrodes).
The blowing out of the discharge spark generated between the central electrode and the ground electrode may be caused by migration of a ground electrode side root of the discharge spark to a ground base member corner, which migration is caused by the air stream in the combustion chamber. That is, the discharge spark is stretched by a mixed fuel-air stream in the combustion chamber and is electrically attracted to the ground base member corner which has high field intensity. This may cause the migration of the ground electrode side root of the discharge spark from a ground protrusion to the ground base member corner. The discharge spark is stretched in a direction such that the air stream is not interrupted by the ground electrode. That is, the discharge spark is stretched in a widthwise direction of a confronting face of the ground base member that confronts the central electrode.
Such migration of the ground electrode side root of the discharge spark to the ground base member corner (referred to as “root migration(s) of discharge spark(s)”) may lead to a higher frequency of re-discharges and thus to an increased amount of wear of the electrodes.
In consideration of the foregoing, exemplary embodiments of the present invention are directed to providing a spark plug capable of preventing the root migrations of discharge sparks and thereby reducing electrode wear.